The known prior art apparatus for sensing luminescence of targets stimulated by direct sunlight and employing Fraunhofer Line sensing technology has been limited to elaborate, bulky, and heavy systems typically built for use in an aircraft or proposed for use in a satellite.
More particularly, the known prior arts constituted the Fraunhofer Line Discriminator (FLD) instrument used for some years by the U.S. Geological Survey (USGS) at Flagstaff, Ariz. and at other locations. This instrument is described in U.S. Pat. No. 3,769,516. Additional U.S. Pat. Nos. 3,578,848; 3,598,994; 3,641,344; and 4,433,245 cover various aspects of the USGS instrument and other contemplated large scale instruments employing Fraunhofer Line sensing technology.
The portable luminescence sensor disclosed herein differs markedly from the airborne/satellite-borne Fraunhofer Line sensing instruments in that it is lightweight and compact and therefore suitable for portable use on the ground by an individual or small team during field exploration and investigative activities.
It is well known in the physical sciences art that the spectrum of sunlight, as it impinges upon the earth, is not uniform but contains many dark lines of narrow bandwidth. The dark lines constituting the absorption spectrum exhibited by sunlight are frequently called Fraunhofer Lines. There are numerous such lines of which Fraunhofer, early in the nineteenth century, first observed the most prominent. To these particular lines he assigned letters for reference purposes. These most prominent lines, together with their designation, origin, and aproximate wavelengths, are listed as follows:
______________________________________ A Terrestrial Oxygen 7594 A Extreme Red B Terrestrial Oxygen 6867 A Red C Solar Hydrogen 6563 A Red D1 Solar Sodium 5896 A Yellow D2 Solar Sodium 5890 A Yellow E Solar Iron 5270 A Green F Solar Hydrogen 4861 A Blue G Solar Iron and Calcium 4308 A Violet H Solar Calcium 3968 A Extreme Violet ______________________________________
The lines of solar origin (all those listed except A and B) are due to absorption by gases and vapors in the solar atmosphere. Similarly, the lines of terrestrial origin (A and B) are due to absorption by gases and vapors in the terrestrial atmosphere. The presence of Fraunhofer Lines in the spectrum is used to sense and measure the luminescence emanating from targets illuminated by direct sunlight.
The fundamental technique utilized to detect and measure luminescence takes advantage of the fact that sunlight, whether direct or reflected, is "coded" by a sharp reduction in spectral energy at certain wavelengths; i.e., at the Fraunhofer lines which are absorption features in the solar spectrum. Luminescence, on the other hand, is not so coded; it has a broad and rather uniform spectral output at least over moderate spectral bandwidths. Thus, a purely reflective scene having no luminescence component will replicate the ratio obtained in pure sunlight of the energy in a narrow band within a selected Fraunhofer Line to the energy in an equal bandwidth in the neighboring continuum; i.e., reflected sunlight is coded. A scene containing luminescence, however, changes the code by disturbing the ratio. As will be explained in more detail below, the energy from the sun in a unit bandwidth proximate the Fraunhofer Line is conventionally designated as quantity "a", while quantity "b" is the corresponding level within the Fraunhofer absorption band. For the earth scene, quantities "d" and "c" represent the corresponding continuum and in-band levels as modified by the scene content if it contains a luminescing component. The quantities a, b, c, and d, all measurable (at least as to their relative intensities), permit the development of equations from which luminescence "L" and refelectance "R" coefficients may be determined. It may be noted that the term "luminescence" as used herein comprehends both fluorescence (which is characterized by rapid decay after the exciting energy source is removed) and phosphorescence (which decays more slowly). In the case of solar stimulated reemission, it is not necessary to be concerned with rapid-time responses nor with distinguishing between fluorescence and phosphorescence.
In their fundamental form, the reflectance (R) and luminescence (L) equations are: EQU R=[(d-c)/(a-b)] EQU L=[(d/a)-R]
Because of the need to measure very precisely the energy within a very narrow Fraunhofer Line to obtain two of the four quantities required to determine luminescence and reflectance, the airborne/satellite-borne systems have universally contemplated the use of extremely narrow filters which, of necessity, are large, heavy, expensive, and must be temperature controlled. Instruments of this type use Fabry-Perot glass spacer etalons. The performance of these filters is well known in the art; however, their aforementioned physical characteristics render them undesirable for use in a Fraunhofer Line sensing instrument intended for handheld use on the ground under field exploration and investigation conditions.
It has been generally thought that a relatively inexpensive, handheld, stable, field-usable instrument employing Fraunhofer Line sensing technology to detect and measure luminescence was not achievable. However, by invention disclosed herein, measurements are taken of the energy within a Fraunhofer Line without resorting to the use of Fabry-Perot etalons or other extremely narrow-band equivalents.